It is useful to use a special camera to acquire visual information on the environment that cannot be acquired by the naked eye so that the visual information that cannot be acquired by the naked eye is used or the visual information by the naked eye is used, depending on the situation. In an application for aiding the operation of a helicopter or an airplane, for example, a helmet with a display (head-mounted display) where a night vision camera is attached to a helmet (mounted body) has been proposed (see Japanese Unexamined Patent Publication H9 (1997)-188911and Japanese Translation of International Unexamined Patent Publication 2001-515150). Such a helmet with a display can display visual information acquired by means of a light-amplifying camera on the helmet visor. As a result, the wearer of the helmet with a display (observer) can clearly see the environment even at night or in similar conditions. A helmet with a display where an infrared camera is attached to a helmet has also been proposed (see Japanese Unexamined Patent Publication H8 (1996)-54282). Such a helmet with a display can display visual information acquired by means of an infrared camera on the helmet visor. Light-amplifying cameras and infrared cameras are generally referred to as night vision cameras, and these terms are used in the present patent.
Here, a display fixed to a helmet is referred to as helmet-mounted display. In addition, a display fixed to a headband (mounted body) is referred to as head-mounted display. Helmet-mounted displays and head-mounted displays are both referred to as HMD (head-mounted display) using the same abbreviation.
HMDs provide the wearer a virtual image (for example, information on speed, direction and altitude) even during the daytime and similar conditions. At this time, it is necessary to allow the wearer to recognize a good image even in the case where it is very bright outside and a large amount of light transmits through the visor, and therefore images having high brightness have been displayed. Concretely, an ultrahigh-intensity CRT (cathode ray tube) has been used as a display element for displaying an image having high brightness. However, the ultrahigh-intensity CRT has such problems that it is heavy and bulky.
Meanwhile, EL elements, which are light and small display elements, and flat panels, such as transmission liquid crystal panels and reflection liquid crystal panels that do not emit light by themselves and use a light source, have been researched, and some of them have been put into practice.
FIG. 9 is a schematic side diagram showing an example of the structure of a helmet with a display using a flat panel, and FIG. 10 is a schematic plan diagram showing the structure of the helmet with a display in FIG. 9.
A helmet with a display 200 is provided with a helmet 3 mounted on the head of a wearer P, a visor 13 placed in front of the wearer P, a night vision camera 21L for the left eye that takes images for the left eye, a night vision camera 21R for the right eye that takes images for the right eye, a display unit 110L for the left eye placed to the upper left of the wearer P, a display unit 110R for the right eye placed to the upper right of the wearer P, a video signal processing circuit unit 22 for controlling the night vision camera 21L for the left eye, the night vision camera 21R for the right eye, the display unit 110L for the left eye and the display unit 110R for the right eye, and a power source, such as a battery, placed in the helmet (not shown).
The helmet 3 is in approximately hemispherical form so as to cover the head of the wearer, and at the same time expose the face.
The visor 13 is in a predetermined form with a curved surface, which is formed of a half mirror or a hologram element. In addition, the visor 13 is supported by the helmet 3 in such a state as being slidable in the up and down directions, and when the visor 13 is lowered, it is located in front of the left eye EL and the right eye ER of the wearer P.
The night vision camera 21L for the left eye and the night vision camera 21R for the right eye have a light-amplifying function, that is, function to amplify the amount of light that enters into the camera, and also function to detect infrared rays radiated from a substance having a high temperature. As a result, an object to be observed can be seen even at night or in similar conditions.
The night vision camera 21L for the left eye and the night vision camera 21R for the right eye are placed to the left and right of the helmet 3, respectively, so as to maintain a certain distance between the cameras in order to see an object to be observed three-dimensionally and at the same time the direction in which lid images are taken is perpendicular to the direction in which the cameras are apart from each other.
Video signals (visual information are inputted from the night vision camera 21L for the left eye and the night vision camera 21R for the right eye into the video signal processing circuit unit 22, which outputs image signals to the display unit 110L for the left, eye and the display unit 1108 for the eye through signal lines 23L and 23R.
In this helmet 200 with a display, light for image display for the left eye emitted from the display unit 110L for the left eye is reflected from the reflecting surface of the visor 13 so as to be lead to the left eye EL of the wearer P, and at the same time light for image display for the right eye emitted from the display unit 110R for the right eye is reflected from the reflecting surface of the visor 13 so as to he lead to the right eye EP of the wearer P, As a result, the wearer P can see a virtual image of the object to be observed three-dimensionally, and at the same time can see the actual things in the environment in the front due to the light that transmits through the visor 13.
FIG. 11 is a schematic side diagram showing an example of the structure of part of the display unit for the left eye in FIGS. 9 and 10.
The display unit 110L for the left eye has a light source 52L, a transmission liquid crystal panel 51L placed in front of the light source 52L, an optical element group 11L placed in front of the transmission liquid crystal panel 51L for projecting a virtual image of the object to be observed, and a signal line 23L for connecting the transmission liquid crystal panel 51L to the video signal processing circuit unit 22.
The transmission liquid crystal panel 51L receives image signals from the video signal processing circuit unit through the signal line 23L so as to generate an image on the flat panel (for example, a one-inch square).
The light source 52L has a number of (65, for example) light emitting diodes. The number of light emitting diodes is arranged two-dimensionally at predetermined intervals. The light emitting diodes emit light to the front. The intensity of light emitted from each light emitting diode is higher at a location closer to the center in front of the light emitting diode (optical axis of the light emitting diode) and lower at a location further a the center. As a result, light is emitted so that the entire surface of the transmission liquid crystal panel 51L is illuminated.
Here, the display unit 110L for the right eye has the same structure as the display unit 110L for the left eye.
Though research has been conducted on helmets 200 with a display as described above in order to increase the intensity of light emitted by the light emitting diodes, images having brightness higher than that of ultrahigh-intensity CRTs cannot be displayed at present, and thus such a problem arises that an image cannot be favorably viewed or recognized in a very bright environment (days with clear weather). Therefore, helmets 200 with a display using a flat panel are limited to indoor use at present.
Though it is possible to supply a large amount of power to the light emitting diodes in order to display an image having high brightness, the light emitting diodes emit heat, and thus the temperature becomes high, and therefore it is not preferable to supply a large amount of power to the light emitting diodes in a helmet 200 with a display that is worn on the head of the wearer P.
Meanwhile, a laser can be used for the light source 521, instead of light emitting diodes so that an image can be favorably viewed and recognized even in a very bright environment. However, the laser is heavy and bulky, and thus the same problems as with ultrahigh-intensity CRTs arise such that a large load is applied to the head of the wearer P.
Thus, an object of the present invention is to provide a head-mounted display that enables a wearer to favorably view and recognize images even in a very bright environment and imposes a small burden on the head of the wearer.